Fine solid particle dispersions of compound useful in imaging can be prepared by mixing together a coarse slurry of a solid compound of interest in a liquid, with or without a dispersing aid and a binder, followed by milling where repeated collisions of milling media with the solid compound in the slurry result in fracture and resultant particle size reduction. The mill used to accomplish particle size reduction can be for example a colloid mill, swinging mill, ball mill, media mill, attritor mill, jet mill, vibratory mill, high pressure homogenizer, etc. These methods are described, e.g., in U.S. Pat. Nos. 4,006,025, 4,294,916, 4,294,917, 4,940,654, 4,950,586 and 4,927,744, and UK 1,570,362. The mill can be charged with the appropriate media such as, for example, sand, spheres of silica, stainless steel, silicon carbide, glass, zirconium, zirconium oxide, alumina, titanium, polymeric media such as cross-linked polystyrene beads, etc. The media sizes typically range from 0.25 to 3.0 mm in diameter, but smaller milling media, e.g. media having a mean particle size less than 100 microns, may also be used. After reduction to a desired particle size, the compound of interest is separated from the milling media. Milling processes may be performed batchwise or in a continuous process manner.
Conventional mills used for size reduction in a continuous mode usually incorporate a means for retaining milling media in the milling zone of the mill (e.g., milling chamber) while allowing passage of the dispersion or slurry through the mill in either a recirculation or discrete pass mode. Such means for simultaneous milling and media separation is described as "dynamic media separation". Various techniques have been established for retaining media in these mills, including rotating gap separators, screens, sieves, centrifugally-assisted screens, and similar devices to physically restrict passage of media from the mill. Batch processes such as ball mills (eg. Abbe Ball Mills) or stirred ball mills (eg. Union Process Attritor) perform separation of dispersion and milling media after milling is complete, usually through a screen or sieve sized smaller than the milling media. Typically, the screen is affixed to the milling vessel and slurry is removed by gravity drainage or pumped out of the vessel. Alternatively, the slurry may be forced from the vessel by charging the vessel with compressed gas.
Over the last ten years there has been a transition to the use of small milling media in conventional media mill processes for the preparation of various paints, pigment dispersions and photographic dispersions. This transition has been made possible due primarily to the improvements in high speed media mill designs (eg. Netzsch LMC mills and Drais DCP mills) which allow the use of media as small as 250 .mu.m. The advantages of small media include more efficient comminution (i.e. faster rates of size reduction) and smaller ultimate particle sizes. While the use of media having a size less than 300 .mu.m, especially between 25 and 100 .mu.m, has been found to provide optimal size reduction as disclosed in copending, commonly assigned U.S. patent application Ser. No. 08/248,774 filed May 25, 1994, the disclosure of which is hereby incorporated by reference, even with the best machine designs available, it has been found difficult to use such media due to separator screen plugging and unacceptable pressure build-up due to hydraulic packing of the media. In fact, for most commercial applications, 350 .mu.m media is considered the practical lower limit for most systems due to media separator screen limitations. In copending, commonly assigned U.S. patent application Ser. No. 08/248,782 filed May 25, 1994, the disclosure of which is hereby incorporated by reference, it is disclosed that the problems of separator screen plugging and unacceptable pressure build up due to hydraulic packing of the media during milling can be avoided by 1) adjustment of the media separator to allow passage of media through the separator, and 2) providing a means of continuous recirculation of the media/product mixture throughout the process, wherein the media and product are separated from each other outside of the milling chamber.